The present invention relates to a method of spraying aerosols which are created during the spraying process from a liquid composition located within a container by forcing the liquid through a suitable spray head attached to the container from which it emerges in aerosol form.
The present invention is particularly concerned with the formation of aerosols using domestic aerosols spray devices. Such devices may contain liquid compositions capable of forming aerosols for use in a wide range of applications including disinfectants, paints, antiperspirants, deodorants and insecticides. Reference will be made hereinafter to disinfectant applications but it should be appreciated that the present invention may be of use in connection with many other aerosol applications.
Dispersion of compositions from an aerosol spray device is not ideal as the spray of liquid droplets emerging from the aerosol device is generally propelled by sufficient force to enable the liquid droplets only to travel more or less in a straight line with a relatively small spread angle for a distance perhaps of the order of 1 meter or more. This is because of the restrictions on the design of the spray head including the outlet orifice. During their travel, the liquid droplets will, until they lose momentum, tend to maintain a straight line path although some attraction may take place between individual liquid droplets causing coalescence between one or more droplets, thereby possibly reducing the spread of droplets from a central spray line extending from the spray head.
In the case of a disinfectant product, the aim of the user is to spray an aerosol disinfectant from the spray device in a manner such that the liquid droplets make contact with microorganisms, typically in the form of bacteria, viruses or fungal spores. Microorganisms tend to accumulate on or adjacent to surfaces which are relatively difficult to access, for example, surfaces located behind the pedestals, handles and rims of sinks, showers, toilets or wash basins. A conventional aerosol spray device, when operated so that the spray is directed in the general direction of these inaccessible areas, will result in the liquid droplets encountering the surfaces of objects which lie between the spray device and the inaccessible areas. Accordingly, the liquid droplets will impinge on these objects and will never make contact with the desired target areas. Furthermore, it is difficult to ensure that a spray of, for example, a disinfectant composition will reach into all of the corners, crevices, pits, hollows and other areas of a surface which are difficult to access. With conventional techniques it is therefore difficult to sterilise the surfaces of operating theatres, hospital wards and other institutions.
We have now developed an improved method of spraying compositions which enables the liquid droplets of the compositions to cover and target surfaces more effectively and, in particular, which enables the droplets to target obscured or partly obscured surfaces.
According, to the present invention there is provided a method of improving the spraying of liquid droplets from a spray device onto a surface which method comprises imparting a unipolar charge to the said liquid droplets by double layer charging during the spraying of the liquid droplets from the spray device, the unipolar charge being at a level such that the said droplets have a charge to mass ratio of at least +/xe2x88x921xc3x9710xe2x88x924 C/kg, whereby the charged droplets of the liquid are mutually repelled thereby increasing the spread of the droplets from a central spray line extending from the head of the spray device and avoiding coalescence of the droplets, thus providing a more even coverage of the surface which is to be sprayed.
In a particular aspect of the present invention there is provided a method of spraying liquid droplets from a spray device onto a surface which is obscured by an object located between the surface and the spray device so that liquid droplets travelling in a straight line from the spray device to the surface will impinge on said object, the method comprising imparting a unipolar charge to the said liquid droplets by double layer charging during the spraying of the liquid droplets from the spray device, the unipolar charge being at a level such that the said droplets have a charge to mass ratio of at least +/xe2x88x921xc3x9710xe2x88x924 C/kg, whereby at least some of the droplets are caused to travel in a path which avoids the object and impinges on the obscured surface.
It will be understood that the object which obscures the surface which it is desired to spray may be a part of the same article as the article possessing the said surface. The invention is thus applicable to the treatment of three dimensional articles where certain surfaces are obscured from View along a particular line of sight. Accordingly, at least a part of the obverse surfaces of articles may be sprayed with liquid droplets using the method of the invention.
It is preferred that the unipolar charge which is imparted to the liquid droplets is generated solely by the interaction between the liquid within the spray device and the spray device itself as the liquid is expelled therefrom. In particular, it is preferred that the manner in which a unipolar charge is imparted to the liquid droplets does not rely even partly upon the connection of the spray device to any external charge inducing device, such as a source of relatively high voltage, or internal charge inducing device, such as a battery. With such an arrangement, the spray device is entirely self-contained, making it suitable for use both in industrial, institutional and domestic situations.
Preferably, the spray device is a domestic pressure-spraying device devoid of any electrical circuitry but which is capable of being hand held. Typically such a device has a capacity in the range of from 10 ml to 2000 ml and can be actuated by hand, or by an automatic actuating mechanism. A particularly preferred domestic device is a hand-held aerosol can.
Preferably, therefore the droplet charge to mass ratio of at least +/xe2x88x921xc3x9710xe2x88x924 C/kg is imparted to the liquid droplets as a result of the use of an aerosol spray device with at least one of the features of the material of the actuator, the size and shape of the orifice of the actuator, the diameter of the dip tube, the characteristics of the valve and the formulation of the composition contained in the aerosol spray device being chosen in order to achieve the said droplet charge to mass ratio by double layer charging imparting the unipolar charge to the droplets during the actual spraying of the liquid droplets from the orifice of the aerosol spray device.
As a result of the method of the present invention surfaces which are normally difficult to access or inaccessible to the spray from an aerosol spray device can be contacted by the spray, thereby rendering many operations much more manageable. By way of example, microorganisms such as bacteria located in areas which are normally difficult to access may be easily eliminated in accordance with the method of the invention.
The method of the present invention may be used, for example, to spray an obscured surface of a sink, shower, toilet, washbasin, chair leg, a handle of a door, cupboard or refrigerator, a part of a human body, or a part of a plant with a liquid composition such as a disinfectant or antimicrobial composition. The method of the invention may also be used to improve the targeting of certain products to their intended destination. For example, more effective spraying of hairsprays, antiperspirants, body sprays, waxes and polishes, oven cleaners, starches and fabric finishes, shoe and leather ware products, glass cleaners, paints, lubricants, house plant sprays, antistick compositions, insecticides, herbicides, fungicides, biopesticides, disinfectants, and various other household, institutional, professional or industrial products may be achieved, with a reduction in the amount of product required to be used and a reduction in the amount of the product which does not impinge upon the target.
The result of the method of the invention is achieved because of the unipolar charge imparted to the liquid droplets of the aerosol spray. This charge has two effects. First, the droplets, since they carried the same polarity charge, are repelled one from another. Accordingly, there is little or no coalescence of the droplets. Rather they tend to spread out to a great extent during their path of travel, thereby following a curved path. In addition, if the repulsive forces from the charge within the droplets is greater than the surface tension force of the droplets, the charged droplets are caused to fragment into a plurality of smaller charged droplets (exceeding the Rayleigh limit). This process continues until either the two opposing forces are equalised or the droplet has fully evaporated.
Secondly, the liquid droplets carrying the unipolar charge are attracted to grounded conducting surfaces such as wood, metal or ceramics by interaction with their image charge. If non-conducting surfaces carry an opposite polarity charge to that of the liquid droplets, then attraction will equally be effected. Those liquid droplets that are attracted to the surface are able to cover the surface much more evenly than with uncharged droplets as at the same time as they are being attracted to the surface, they are also repelling one another.
Accordingly, those liquid droplets which proceed past the obscuring object and are not travelling in a direction towards the target surface, will tend to be attracted towards the target surface and therefore their path of travel follows a curve terminating at the surface.
It will be appreciated that the two effects can be cumulative such that some at least of the liquid droplets will follow a significantly curved path of travel from the aerosol spray device, past the obscuring object and into contact with the target surface.
The liquid composition which is sprayed into the air using the aerosol spray device is preferably a water and hydrocarbon mixture, or emulsion, or a liquid which is converted into an emulsion by shaking the spraying device before use, or during the spraying process.
Whilst all liquid aerosols are known to carry a net negative or positive charge as a result of double layer charging, or the fragmentation of liquid droplets, the charge imparted to droplets of liquid sprayed from standard devices is only of the order of +/xe2x88x921xc3x9710xe2x88x928 to 1xc3x9710xe2x88x925 C/kg.
The invention relies on combining various characteristics of the design of an aerosol spray device so as to increase the charging of the liquid as it is sprayed from the aerosol spray device.
A typical aerosol spray device comprises:
1. An aerosol can containing the composition to be sprayed from the device and a liquid or gaseous propellant.
2. A dip tube extending into the can, the upper end of the dip tube being connected to a valve;
3. An actuator situated above the valve which is capable of being depressed in order to operate the valve; and
4. An insert provided in the actuator comprising an orifice from which the composition is sprayed.
A preferred aerosol spray device for use in the present invention is described in WO 97/12227.
It is possible to impart higher charges to the liquid droplets by choosing aspects of the aerosol device including the material, shape and dimensions of the actuator, the actuator insert, the valve and the diptube and the characteristics of the liquid which is to be sprayed, so that the required level of charge is generated as the liquid is dispersed as droplets.
A number of characteristics of the aerosol system increase double layer charging and charge exchange between the liquid formulation and the surfaces of the aerosol system. Such increases are brought about by factors which may increase the turbulence of the flow through the system, and increase the frequency and velocity of contact between the liquid and the internal surfaces of the container and valve and actuator system.
By way of example, characteristics of the actuator can be optimised to increase the charge levels on the liquid sprayed from the container. A smaller orifice in the actuator insert, of a size of 0.45 mm or less, increases the charge level of the liquid sprayed through the actuator. The choice of material for the actuator can also increase the charge level on the liquid sprayed from the device with material such as nylon, polyester, acetal, PVC and polypropylene tending to increase the charge level. The geometry of the orifice in the insert can be optimised to increase the charge level on the liquid as it is sprayed through the actuator. Inserts which promote the mechanical break-up of the liquid give better charging.
The actuator insert of the spray device may be formed from a conducting, insulating, semi-conducting or static-dissipative material.
The characteristics of the dip tube can be optimised to increase the charge level in the liquid sprayed from the container. A narrow dip tube, of for example about 1.27 mm internal diameter, increases the charge level on the liquid, and the dip tube material can also be changed to increase charge.
Valve characteristics can be selected which increase the charge to mass ratio of the liquid product as it is sprayed from the container. A small tailpiece orifice in the housing, of about 0.65 mm, increases product charge to mass ratio during spraying. A reduced number of holes in the stem, for example 2xc3x970.50 mm, also increase product charge during spray. The presence of a vapour phase tap helps to maximise the charge level, a larger orifice vapour phase tap of, for example, about 0.50 mm to 1.0 mm generally giving higher charge level.
Changes in the product formulation can also affect charging levels. A formulation containing a mixture of hydrocarbon and water, or an emulsion of an immiscible hydrocarbon and water, will carry a higher charge to mass ratio when sprayed from the aerosol device than either a water alone or hydrocarbon alone formulation.
It is preferred that an aerosol spray composition of use in the present invention comprises an oil phase, an aqueous phase, a surfactant and a propellant.
Preferably the oil phase includes C9-C12 hydrocarbon which is preferably present in the composition in the amount of from 2 to 10% w/w.
Preferably the surfactant is glyceryl oleate or a polyglycerol oleate, preferably present in the composition in an amount of from 0.1 to 1.0% w/w.
Preferably the propellant is liquified petroleum gas (LPG) which is preferably butane, optionally in admixture with propane. The propellant may be present in an amount of from 10 to 90% w/w depending upon whether the composition is intended for spraying as a xe2x80x9cwetxe2x80x9d or as a xe2x80x9cdryxe2x80x9d composition. For a xe2x80x9cwetxe2x80x9d composition, the propellant is preferably present in an amount of from 20 to 50% w/w, more preferably in an amount of from 30 to 40% w/w.
The liquid droplets sprayed from the aerosol spray device will generally have diameters in the range of from 5 to 100 micrometers, with a peak of droplets of about 40 micrometers. The liquid which is sprayed from the aerosol spray device may contain a predetermined amount of a particulate material, for example, fumed silica, or a predetermined amount of a volatile solid material, such as menthol or naphthalene.
A can for a typical aerosol spray device is formed of aluminium or lacquered or unlacquered tin plate or the like. The actuator insert may be formed or, for instance, acetal resin. The valve stem lateral opening may typically be in the form of two apertures of diameters 0.51 mm.